Optical spectrometers allow the study of a large variety of samples over a wide range of wavelengths. Materials can be studied in the solid, liquid, or gas phase either in a pure form or in mixtures. Various designs allow the study of spectra as a function of temperature, pressure, and external magnetic fields.
Grating spectrometers, in particular, make use of the diffraction of light from a regularly spaced ruled surface. They disperse the light by a combination of diffraction and interference rather than the refractive index variation with wavelength. The normal operation of a grating is the same as with a prism. The grating is rotated, and wavelength after wavelength passes a field stop and is detected by a sensor. In general, a grating spectrometer operates by focusing the light through an optical system to the field stop. In a classical spectrometer the field stop is a slit. This light is then collimated and passes through a transmission grating or passed to a reflective grating. The dispersed light is then either focused onto a spectral array or through an exit slit to a detector where it can be analyzed. While plane gratings require separate collimating optics, concave gratings combine the function of the grating and collimating optics into a single optical component.
Near-Infrared (NIR) spectroscopy is one of the most rapidly growing methodologies in pharmaceutical analysis. In particular, NIR is being increasingly used as an inspection method during the packaging process of pharmaceuticals, often augmenting or replacing previously used vision inspection systems. For example, an NIR inspection system can be used to inspect a blister packaging for, among other things, proper filling, physical aberrations, chemical composition, moisture content, and proper package arrangement.
The use of vision systems as an inspection mechanism is becoming less and less sufficient as the need for more in depth inspection procedures, and near 100% inspection processes, are desired and in many cases required. Of particular note is that vision systems are not capable of performing any sort of chemical analysis of the product being packaged, relying only on a comparison of a visual snapshot of the package to a reference image. A typical vision packaging inspection system “looks” at each individual package to see whether it has the correct number of doses in the pack, i.e. the system looks for missing or overfilled tablet wells. In some cases, physical discrepancies such as cracks or gouges on a tablet, will also cause a rejection of the package. The limitations of these types of vision systems become apparent when they are compared with the capabilities of a spectrometer adapted to function in a pharmaceutical packaging and inspection facility.
In high speed, large-volume processing, automated spectrometer-based monitoring systems have become indispensable in examining product flow in order to detect irregularities. Since these systems are meant in large part to replace vision systems, accuracy is a critical factor.
Known spectrometer designs typically incorporate a single entrance slit (field stop) and a single exit slit. The single exit slit typically corresponds to a single detector or other sensor and the measurement system requires a separate spectrometer (including entrance slit, exit slit, and grating) for each required simultaneous spectrum measurement. When multiple simultaneous spectrum measurements are desired, the cost and complexity of a spectrometer system capable of performing such analysis increases dramatically, particularly because of the need for multiple gratings. What is needed is a device and method that provides for multiple and simultaneous spectrum measurements while only requiring a single spectrometer.